Separator and separator seal for polymer electrolyte fuel cells

ABSTRACT

A rubber composition comprising (A) a liquid alkenyl-containing organopolysiloxane with Mw&lt;100,000, (B) a gum-like organopolysiloxane with Mw≧150,000, (C) an organohydrogenpolysiloxane, (D) fumed silica, and (E) an addition reaction catalyst and having a viscosity of 20-200 Pa-s at a shear rate of 10 s −1  and 25° C. is effectively injection moldable into a cured product which is useful as a separator seal in PEFCs.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2007-317475 filed in Japan on Dec. 7, 2007,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to separators and separator seals for use inpolymer electrolyte fuel cells (PEFC) featuring compactness and moreparticularly, to separators and separator seals for PEFC which have along service life and ease of molding.

BACKGROUND ART

The fuel cell is capable of producing electricity without a substantialneed for fossil fuel that poses concerns about resource depletion,without noise, and at a high energy recovery rate as compared with otherenergy-based power generating systems. Great efforts have been made toexploit the fuel cell as a power generating plant of relatively compactsize in buildings and factories, with some cells having beencommercially implemented. In particular, polymer electrolyte fuel cells(PEFC) can operate at lower temperature than fuel cells of other types.The PEFC then draws attention not only as a device for householdco-generation, but also as the replacement power source for internalcombustion engines on vehicles because of the minimized corrosionconcern regarding the materials of which cell components are made andtheir ability to discharge relatively high current flow despite lowtemperature operation. The PEFC is constructed of electrolyte membranes,separators and other components. The separator is generally a platewhich is provided with a plurality of parallel channels on one surfaceor both surfaces. The separator plays the role of conducting theelectricity produced at the gas diffusion electrode within the fuel cellto the exterior, discharging water produced within the channels in thecourse of electricity generation, and securing the channels as a flowpath for incoming reaction gas to the fuel cell. Such a fuel cellseparator is required to be more compact in size. Since a multiplicityof separators are used in stack, there is a demand for a separator sealmaterial having durability and long term service.

As the separator sealing material, packing materials based on variousresins have been under study. Among them, sealing materials based onsilicone rubber are often used for their moldability, heat resistanceand elasticity. JP-A 11-129396, JP-A 11-309747 and JP-A 2001-58338disclose liquid silicone rubber compositions of the addition cure typehaving a good flow and brief cure. These compositions are injectionmoldable to form seals. However, silicone rubbers obtained by curing theaddition cure type compositions are still unsatisfactory in maintainingelasticity over a long term.

JP-A 2002-313373 and JP-A 2003-257455 disclose organopolysiloxanecompositions with reduced compression set. For the seal on fuel cellseparators, in particular, seal performance in acidic aqueous solutionis important. JP-A 2002-309092 discloses how to reduce the compressionset in acidic aqueous solution.

For the PEFC, several tens to several hundreds of unit cells whereinthin-film structures having an electrolyte membrane sandwiched betweenelectrodes are alternated with separators and assembled into a stack. Inorder that the overall stack be compact, it is necessary that separatorsubstrates have a thickness of up to 1 mm and seals formed thereon be asthin as 0.5 mm or less, for example. When such thin-wall seals areformed on separator substrates, the flow and rubber strength of theseal-forming material become important factors. At the same time, acidresistance and compression set are also requisite as the seal. However,JP-A 11-129396 and JP-A 11-309747 cited above merely describe anappropriate viscosity range of 1,000 to 10,000 poise, but refer nowhereto the detail of silicone rubber. While the flow at a high shear is akey for injection molding, these patents do not discuss the viscosityversus shear rate. In our experiment, the silicone resin KE1950-60A/B(Shin-Etsu Chemical Co., Ltd.) used in Examples was found to have aviscosity in excess of 200 Pa-s at a shear rate of 10 s⁻¹, which is notregarded appropriate to form thin-wall seals. JP-A 2001-58338 describesan example in which a liquid silicone rubber material is injectionmolded to form a seal, but refers nowhere to the composition and flow ofthe material.

JP-A 2002-313373 discloses a composition comprising fumed silica treatedwith two different surface treating agents, but refers nowhere to theviscosity and moldability of the composition. JP-A 2003-257455 describesthat rubber having improved compression set and strength is obtainableby the combined use of two different alkenyl-containing fluids, butrefers nowhere to moldability.

SUMMARY OF THE INVENTION

An object of the invention is to provide a separator seal for use inpolymer electrolyte fuel cells, which is made of cured rubber havingease of molding, reduced compression set, and satisfactory sealperformance; and a separator having the seal formed at the periphery ofa separator substrate.

The inventors have found that a liquid silicone rubber compositioncomprising as essential components (A) a liquid organopolysiloxanecontaining at least two alkenyl groups each attached to a silicon atomin a molecule and having a weight average molecular weight of less than100,000, (B) a gum-like organopolysiloxane having a weight averagemolecular weight of at least 150,000, (C) an organohydrogenpolysiloxanecontaining at least three silicon-bonded hydrogen atoms in a molecule,(D) fumed silica having a BET specific surface area of 50 to 400 m²/g,and (E) an addition reaction catalyst is effectively moldable,especially injection moldable, when the composition has a viscosity of20 to 200 Pa-s at a shear rate of 10 s⁻¹ and 25° C., that the curedrubber has reduced compression set and exhibits excellent sealperformance so that it becomes an effective seal on a separator inPEFCs.

Accordingly, in one aspect, the present invention provides a separatorseal for use in polymer electrolyte fuel cells which is formed of asealing composition in the cured state, said sealing compositioncomprising as essential components,

(A) 100 parts by weight of a liquid organopolysiloxane containing atleast two alkenyl groups each attached to a silicon atom in a moleculeand having a weight average molecular weight of less than 100,000,

(B) 3 to 30 parts by weight of a gum-like organopolysiloxane having aweight average molecular weight of at least 150,000,

(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxanecontaining at least three hydrogen atoms each attached to a silicon atomin a molecule,

(D) 5 to 30 parts by weight of fumed silica having a BET specificsurface area of 50 to 400 m²/g, and

(E) a catalytic amount of an addition reaction catalyst,

said composition having a viscosity of 20 to 200 Pa-s at 25° C. and ashear rate of 10 s⁻¹.

In a preferred embodiment, the alkenyl-containing organopolysiloxane (A)has a weight average molecular weight of 10,000 to 80,000, at least 90mol % of the entire organic groups attached to silicon atoms beingmethyl. Also preferably, a molar ratio of Si—H groups in component (C)to total alkenyl groups in components (A) and (B) is between 0.8:1 and5.0:1. In a preferred embodiment, the composition at 25° C. has aviscosity V(10) at a shear rate of 10 s⁻¹ and a viscosity V(0.9) at ashear rate of 0.9 s⁻¹, which fall in the range: 1.0<V(0.9)/V(10)<2.5.Typically, the gum-like organopolysiloxane (B) contains alkenyl groupsin an amount of 1.0×10⁻⁶ to 1.0×10⁻³ mol/g.

In another aspect, the present invention provides a separator for use inPEFCs comprising a substrate comprising a metal thin plate or aconductive powder and a binder and a seal formed at a periphery on atleast one surface of the substrate, the seal comprising a cured productof the sealing composition.

BENEFITS OF THE INVENTION

The sealing composition of the invention is effectively moldable,especially injection moldable. The cured rubber has reduced compressionset and exhibits excellent seal performance so that it becomes aneffective separator seal for use in PEFCs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective exploded view showing main components of apower-generating cell in a fuel cell stack according to one embodimentof the invention.

FIG. 2 is a cross-sectional view of the fuel cell stack taken alonglines II-II in FIG. 1.

FIG. 3 is a cross-sectional view traversing a fuel gas inletcommunication hole of the fuel cell stack.

FIG. 4 is a cross-sectional view traversing an oxidant gas inletcommunication hole of the fuel cell stack.

FIG. 5 is a front view of a first metal separator constituting thepower-generating cell.

FIG. 6 is a front view showing one surface of a second metal separatorconstituting the power-generating cell.

FIG. 7 is a front view showing the other surface of the second metalseparator constituting the power-generating cell.

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, the singular forms “a,” “an” and “the” do not denote alimitation of quantity, but rather denote the presence of at least oneof the referenced item. The terms “first,” “second” and the like do notdenote any order or importance, but rather are used to distinguish oneelement from another. It is also understood that terms such as “top,”“bottom,” “outward,” “inward” and the like are words of convenience andare not to be construed as limiting terms.

The sealing material or sealing rubber composition of the invention isused to form a seal at the periphery on at least one surface of aseparator for use in polymer electrolyte fuel cells (PEFCs). The sealingcomposition comprises as essential components,

(A) 100 parts by weight of a liquid organopolysiloxane containing atleast two alkenyl groups each attached to a silicon atom in a moleculeand having a weight average molecular weight of less than 100,000,

(B) 3 to 30 parts by weight of a gum-like organopolysiloxane having aweight average molecular weight of at least 150,000,

(C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxanecontaining at least three hydrogen atoms each attached to a silicon atomin a molecule,

(D) 5 to 30 parts by weight of fumed silica having a BET specificsurface area of 50 to 400 m²/g, and

(E) a catalytic amount of an addition reaction catalyst.

Component A

Component (A) is a liquid organopolysiloxane containing at least twoalkenyl groups each attached to a silicon atom in a molecule and havinga weight average molecular weight of less than 100,000. Most often, itis represented by the following average compositional formula (I):

R¹ _(a)SiO_((4−a)/2)   (I)

wherein R¹ which may be the same or different is a substituted orunsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms,preferably 1 to 8 carbon atoms, and “a” is a positive number in therange of 1.5 to 2.8, preferably 1.8 to 2.5.

Examples of the substituted or unsubstituted monovalent hydrocarbongroup represented by R¹ include alkyl groups such as methyl, ethyl,propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl,hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups such as phenyl,tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyland phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl,isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, as well assubstituted forms of the foregoing groups in which some or all hydrogenatoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro),cyano groups or the like, such as chloromethyl, chloropropyl,bromoethyl, trifluoropropyl and cyanoethyl. Preferably, at least 90 mol% of the entire R¹ are methyl.

At least two of R¹ should be alkenyl groups, preferably of 2 to 8 carbonatoms, more preferably 2 to 6 carbon atoms, and most preferably vinyl.The content of alkenyl groups is preferably 5.0×10⁻⁶ mol/g to 5.0×10⁻³mol/g, more preferably 1.0×10⁻⁵ mol/g to 1.0×10⁻³ mol/g of theorganopolysiloxane. An alkenyl content of less than 5.0×10⁻⁶ mol/g maygive too low a rubber hardness to provide a satisfactory seal whereas analkenyl content of more than 5.0×10⁻³ mol/g may result in a highercrosslinked density and hence, brittle rubber.

The alkenyl groups may be attached to a silicon atom at the end of themolecular chain or a silicon atom midway the molecular chain or both.The inclusion of at least alkenyl groups attached to silicon atoms atboth ends of the molecular chain is preferred.

The preferred organopolysiloxane basically has a linear structure, butmay partially have a branched or cyclic structure.

In the invention, the alkenyl-containing organopolysiloxane as component(A) should have a weight average molecular weight Mw of less than100,000, and preferably in the range of 10,000 to 80,000. If Mw is lessthan 10,000, the resulting sealing composition may not have satisfactoryrubber elasticity. If Mw is more than 100,000, the resulting sealingcomposition becomes too viscous to mold. As used herein, the weightaverage molecular weight is measured by gel permeation chromatography(GPC) versus polystyrene standards.

Component B

Component (B) is an organopolysiloxane having a weight average molecularweight of at least 150,000 which is gum-like at room temperature. Mostoften, it is represented by the following average compositional formula(II):

R² _(b)SiO_((4−b)/2)   (II)

wherein R² which may be the same or different is a substituted orunsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms,preferably 1 to 8 carbon atoms, and “b” is a positive number in therange of 1.5 to 2.8, preferably 1.8 to 2.5.

Examples of the substituted or unsubstituted monovalent hydrocarbongroup represented by R² include alkyl groups such as methyl, ethyl,propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl,hexyl, cyclohexyl, octyl, nonyl, and decyl; aryl groups such as phenyl,tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyland phenylpropyl; alkenyl groups such as vinyl, allyl, propenyl,isopropenyl, butenyl, hexenyl, cyclohexenyl and octenyl, as well assubstituted forms of the foregoing groups in which some or all hydrogenatoms are replaced by halogen atoms (e.g., fluoro, bromo and chloro),cyano groups or the like, such as chloromethyl, chloropropyl,bromoethyl, trifluoropropyl and cyanoethyl. Preferably, at least 90 mol% of the entire R² are methyl.

R² may or may not contain alkenyl groups although the inclusion ofalkenyl groups is preferred. The alkenyl groups, if present, preferablyhave 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, and mostpreferably vinyl. The content of alkenyl groups is preferably 1.0×10⁻⁶mol/g to 1.0×10⁻³ mol/g, more preferably 1.0×10⁻⁵ mol/g to 5.0×10⁻⁴mol/g of the organopolysiloxane. An alkenyl content of more than1.0×10⁻³ mol/g may result in a higher crosslinked density and hence,brittle rubber or aggravated compression set.

The alkenyl groups may be attached to a silicon atom at the end of themolecular chain or a silicon atom midway the molecular chain or both.

The preferred organopolysiloxane basically has a linear structure, butmay partially have a branched or cyclic structure.

In the invention, the organopolysiloxane as component (B) should have aweight average molecular weight Mw equal to or more than 150,000.Although the upper limit need not be specified, it is preferred forviscosity and molding after mixing that Mw fall in the range of 200,000to 1,000,000.

The gum-like organopolysiloxane (B) is blended in an amount of 3 to 30parts, preferably 4 to 25 parts, and more preferably 5 to 20 parts byweight per 100 parts by weight of component (A).

Component C

Component (C) is an organohydrogenpolysiloxane containing at least threehydrogen atoms each attached to a silicon atom (i.e., Si—H groups) in amolecule. It serves as a crosslinking agent for curing the compositionwherein Si—H groups in the molecule crosslink with silicon-bondedalkenyl groups in components (A) and (B) through hydrosilylationaddition reaction.

Most often, the organohydrogenpolysiloxane (C) is represented by thefollowing average compositional formula (III):

R³ _(c)H_(a)—SiO_((4−c−d)/2)   (III)

wherein R³ is each independently a substituted or unsubstitutedmonovalent hydrocarbon group having 1 to 10 carbon atoms, preferably 1to 8 carbon atoms, “c” is a positive number of 0.7 to 2.1, “d” is apositive number of 0.001 to 1.0, and c+d is from 0.8 to 3.0. Preferredare those of formula (III) having at least three (typically 3 to 300),more preferably 3 to 100, most preferably 3 to 50 silicon-bondedhydrogen atoms in a molecule.

Examples of the monovalent hydrocarbon group represented by R³ are asexemplified above for R¹ although groups free of aliphatic unsaturation(as in alkenyl groups) are preferred. Preferably, “c” is 0.8 to 2.0, “d”is 0.01 to 1.0, and c+d is from 1.0 to 2.5.

The molecular structure of the organohydrogenpolysiloxane may bestraight, cyclic, branched or three-dimensional network. The number ofsilicon atoms per molecule or the degree of polymerization is preferablyabout 2 to about 300, especially about 4 to about 150. Differentlystated, the preferred organohydrogenpolysiloxanes are those which areliquid at room temperature (25° C.) and specifically have a viscosity ofup to 1,000 mPa-s, and more preferably 0.1 to 500 mPa-s at 25° C. asmeasured by a rotational viscometer.

The hydrogen atoms may be attached to a silicon atom at the end of themolecular chain or a silicon atom midway the molecular chain or both.

Exemplary of the organohydrogenpolysiloxane (C) aretrimethylsiloxy-endcapped methylhydrogenpolysiloxane,trimethylsiloxy-endcapped dimethylsiloxane-methylhydrogensiloxanecopolymers, dimethylhydrogensiloxy-endcapped dimethylpolysiloxane,dimethylhydrogensiloxy-endcapped dimethylsiloxane-methylhydrogensiloxanecopolymers, trimethylsiloxy-endcappedmethylhydrogensiloxane-diphenylsiloxane copolymers,trimethylsiloxy-endcappedmethylhydrogensiloxane-diphenylsiloxane-dimethylsiloxane copolymers,copolymers composed of (CH₃)₂HSiO_(1/2) units and SiO_(4/2) units, andcopolymers composed of (CH₃)₂HSiO_(1/2) units, SiO_(4/2) units and(C₆H₅)SiO_(3/2) units. As used herein, the term “endcapped” means thatthe polysiloxane is capped at both ends of its molecular chain with theindicated groups.

The amount of the organohydrogenpolysiloxane (C) blended is 0.5 to 20parts, and preferably 0.6 to 15 parts by weight, per 100 parts by weightof component (A). The molar ratio of silicon-bonded hydrogen atoms (Si—Hgroups) in component (C) to total alkenyl groups in components (A) and(B), [Si—H/alkenyl], is preferably from 0.8:1 to 5.0:1, especially from1.0:1 to 3.0:1. A molar ratio outside this range may lead to curedrubber with increased compression set, aggravating the seal performance.

Component D

Component (D) is fumed silica which is essential to impart satisfactorystrength to silicone rubber. The fumed silica should have a specificsurface area of 50 to 400 m²/g, and preferably 100 to 350 m²/g, asmeasured by the BET method. A surface area below 50 m²/g may compromiseacid resistance whereas above 400 m²/g, compression set increases. Thefumed silica may be used as such, but preferably after treatment with asurface hydrophobizing agent. Alternatively, a surface treating agent isadded when the fumed silica is mixed with the silicone fluid, wherebythe fumed silica is treated during the mixing step. Suitable surfacetreating agents are well-known agents including alkylalkoxysilanes,alkylchlorosilanes, alkylsilazanes, silane coupling agents, titanatetreating agents, and fatty acid esters alone or in admixture. When twoor more agents are used, they may be applied at the same time ordifferent times.

The amount of fumed silica (D) blended is 5 to 30 parts, especially 10to 28 parts by weight, per 100 parts by weight of component (A). Thepreferred amount of fumed silica is 15 to 28 parts when it has a surfacearea of 50 to 180 m²/g, and 10 to 20 parts when it has a surface area of180 to 400 m²/g. Less than 5 parts of the fumed silica fails to providesatisfactory rubber strength whereas more than 30 parts increasescompression set, aggravating the seal performance.

Component E

Component (E) is an addition reaction catalyst for promoting additionreaction between alkenyl groups in the organopolysiloxanes as components(A) and (B) and silicon-bonded hydrogen atoms (Si—H groups) in theorganohydrogenpolysiloxane as component (C). Most often, the catalyst isselected from platinum group metal-based catalysts including platinumcatalysts such as platinum black, platinic chloride, chloroplatinicacid, reaction products of chloroplatinic acid with monohydric alcohols,complexes of chloroplatinic acid with olefins, and platinumbisacetoacetate as well as palladium catalysts and rhodium catalysts,with the platinum catalysts being preferred.

The amount of the catalyst blended is a catalytic amount to promoteaddition reaction and usually about 0.5 to 1,000 ppm, especially about 1to 500 ppm of platinum group metal based on the weight of component (A).Less than 0.5 ppm may be ineffective to promote addition reaction,leading to undercure. Amounts of more than 1,000 ppm may exert littlefurther effect on reactivity and be uneconomical.

Other components

If necessary, the composition may further contain other components, forexample, fillers such as precipitated silica, ground quartz,diatomaceous earth and calcium carbonate; hydrosilylation reactionregulating agents such as nitrogen-containing compounds, acetylenecompounds, phosphorus compounds, nitrile compounds, carboxylates, tincompounds, mercury compounds, and sulfur compounds; heat resistanceimprovers such as iron oxide and cerium oxide; internal parting agentssuch as dimethylsilicone fluid; tackifiers, and thixotropic agents.

Separator Seal

The separator seal is formed of the sealing material or additionreaction cure type silicone rubber composition comprising theabove-described components in the cured state. The silicone rubbercomposition may be applied and cured by well-known techniques, forming aseal on a PEFC separator.

More particularly, when PEFC separator seals are manufactured using thecured rubber, the silicone rubber composition is molded into a sealshape by a compression molding, casting or injection molding technique,and the molded seal is combined with a separator substrate.Alternatively, integrated seal-separator substrate members aremanufactured by dipping, coating, screen printing, or insert molding. Ofthese molding methods, the method involving holding a separatorsubstrate in place in a mold and injection molding the rubbercomposition into the mold cavity on one side or both sides of thesubstrate, commonly referred to as insert molding method, is mostpreferred because a thin-wall seal can be briefly formed at a highaccuracy. In the practice of insert molding, the fluidity of the rubbercomposition is important. In order that a seal having a thickness of upto 0.5 mm be insert molded at a high accuracy and efficiency, the rubbercomposition should have a fluidity corresponding to a viscosity of 20 to200 Pa-s at a shear rate of 10 s⁻¹ and 25° C., preferably 25 to 150Pa-s, and more preferably 30 to 120 Pa-s at a shear rate of 10 s⁻¹ and25° C. If the viscosity is less than 20 Pa-s, it is difficult to controlburrs. If the viscosity is more than 200 Pa-s, the material gives riseto such problems as scorching and weld-marks during molding. In apreferred embodiment, the composition at 25° C. has a viscosity V(10) ata shear rate of 10 s⁻¹ and a viscosity V(0.9) at a shear rate of 0.9s⁻¹, which fall in the range: 1.0<V(0.9)/V(10)<2.5, and more preferably1.0<V(0.9)/V(10)<2.0. If this value is less than 1.0 or more than 2.5,the composition may be difficult to mold. It is noted that the viscosityas used herein may be measured by a precision rotational viscometerRotovisco RV1 by Eko Instruments Co., Ltd.

Preferred curing conditions for the silicone rubber composition includea temperature of 100 to 300° C. and a time of 10 seconds to 30 minutesand more preferably 120 to 200° C. and 15 seconds to 5 minutes. A timeof less than 10 seconds is too short for rubber curing whereas more than30 minutes is uneconomical. The cured rubber may be post-cured in anoven for the purposes of reducing compression set or the like. Preferredpost-curing conditions include a temperature of 100 to 220° C. and atime of 30 minutes to 100 hours and more preferably 120 to 200° C. and 1to 24 hours.

The separator substrate used herein may be a metal thin plate or asubstrate manufactured by integral molding of an electrically conductivepowder in a binder. A seal is formed from the silicone rubbercomposition along the periphery of this separator substrate by theabove-described technique, whereupon a PEFC separator within the scopeof the invention is available.

Examples of the conductive powder include natural graphite such as flakegraphite, artificial graphite, and conductive carbon blacks such asacetylene black and Ketjen Black. Any powders may be used as long asthey are electrically conductive. Suitable binders include epoxy resins,phenolic resins, and rubber-modified phenolic resins.

According to the invention, the silicone rubber composition is appliedand cured to the periphery of a separator substrate by a suitabletechnique such as compression molding, casting, injection molding,transfer molding, dipping, coating or screen printing. Thus the curedsilicone rubber composition forms a seal. This results in a separatorfor PEFCs in which a ring-like seal (separator seal) is formed on thesubstrate so as to circumferentially extend along the periphery of thesubstrate.

Preferably the seal has a thickness or height of 0.1 to 2 mm. A seal ofless than 0.1 mm may be difficult to form and exert less sealing effectswhereas a seal of more than 2 mm may be inconvenient for size reduction.

Now referring to the figures, some embodiments of the separator forPEFCs according to the invention are illustrated. The invention is notlimited thereto.

FIG. 1 is a perspective exploded view showing main components of apower-generating cell 12 constituting a fuel cell stack 10 according toone embodiment of the invention. A plurality of power-generating cells12 are stacked in the direction of arrow A to construct the fuel cellstack 10. FIG. 2 is a cross-sectional view of this fuel cell stack 10taken along lines II-II in FIG. 1.

As shown in FIGS. 2 to 4, the fuel cell stack 10 includes a plurality ofpower-generating cells 12 stacked in the direction of arrow A and endplates 14 a, 14 b at opposite ends in the stacking direction. The endplates 14 a, 14 b are fixedly tied via tie rods (not shown) so that afastening load is applied across the stacked cells 12 in the directionof arrow A.

As shown in FIG. 1, each power-generating cell 12 includes anelectrolyte membrane-electrode assembly (MEA) 16 interposed betweenfirst and second metal separators 18 and 20. The first and second metalseparators 18 and 20 are, for example, steel plates, stainless steelplates, aluminum plates, plated steel plates or such metal plates whichhave been surface treated to be corrosion resistant. Their thickness isset in the range of 0.05 to 1.0 mm, for example.

At one side edge of the power-generating cell 12 in the direction ofarrow B (in FIG. 1, typically horizontal direction), an oxidant gasinlet communication hole 30 a for feeding an oxidant gas such asoxygen-containing gas, a coolant outlet communication hole 32 b fordischarging a coolant medium, and a fuel gas outlet communication hole34 b for discharging a fuel gas such as hydrogen-containing gas, whichare in fluid communication with corresponding holes in adjacent cells inthe direction of arrow A or stacking direction, are arranged in thedirection of arrow C (typically vertical direction).

At the other side edge of the power-generating cell 12 in the directionof arrow B, a fuel gas inlet communication hole 34 a for feeding thefuel gas, a coolant inlet communication hole 32 a for feeding thecoolant medium, and an oxidant gas outlet communication hole 30 b fordischarging the oxidant gas, which are in fluid communication withcorresponding holes in adjacent cells in the direction of arrow A, arearranged in the direction of arrow C.

Specifically, the MEA 16 includes a solid polymer electrolyte membrane36 in the form of a perfluorocarbon sulfonic acid thin film impregnatedwith water, which is sandwiched between an anode (or first electrode) 38and a cathode (or second electrode) 40. The anode 38 has a smallersurface area than the cathode 40.

The anode 38 and cathode 40 each include a gas diffusion layer formed ofcarbon paper or the like, and an electrocatalytic layer which is formedby uniformly applying porous carbon particles having a platinum alloysupported on their surfaces to the surface of the gas diffusion layer.The electrocatalytic layers are joined to the opposite surfaces of thesolid polymer electrolyte membrane 36.

The first and second metal separators 18 and 20 have inner surfaces 18 aand 20 a facing MEA 16 and outer surfaces 18 b and 20 b, respectively.The inner surface 18 a of first metal separator 18 is provided withoxidant gas flow channels (reaction gas flow channels) 42 which extendin a serpentine manner in the direction of arrow B and vertically upward(see FIGS. 1 and 5). As shown in FIG. 6, the inner surface 20a of secondmetal separator 20 is provided with fuel gas flow channels (reaction gasflow channels) 44 which are in fluid communication with fuel gas inletcommunication hole 34 a and fuel gas outlet communication hole 34 b aswill be described later, and extend in a serpentine manner in thedirection of arrow B and vertically upward (in the direction of arrowC).

As shown in FIGS. 1 and 2, coolant flow channels 46 are defined betweenthe surfaces 18 b and 20 b of first and second metal separators 18 and20 and in fluid communication with coolant inlet and outletcommunication holes 32 a and 32 b. The coolant flow channels 46 extendstraight in the direction of arrow B.

As shown in FIGS. 1 to 5, a first seal member 50 extendscircumferentially along the peripheral edge of first metal separator 18and is integrally joined to the surfaces 18 a and 18 b of first metalseparator 18. The first seal member 50 is formed by applying the rubbercomposition to the separator substrate by a technique such ascompression molding, casting, injection molding, transfer molding,dipping, coating or screen printing, and curing.

The first seal member 50 includes a first planar portion 52 which isintegrally joined to the surface 18 a of first metal separator 18 and asecond planar portion 54 which is integrally joined to the surface 18 bof first metal separator 18. The second planar portion 54 extends longerthan the first planar portion 52.

As shown in FIGS. 2 and 3, the first planar portion 52 extendscircumferentially at a position outward spaced apart from the peripheraledge of MEA 16, and the second planar portion 54 extendscircumferentially over a region overlying a certain portion of cathode40. As shown in FIG. 5, the first planar portion 52 is formed such thatoxidant gas inlet and outlet communication holes 30 a and 30 b are influid communication with oxidant gas flow channels 42, and the secondplanar portion 54 is formed such that coolant inlet communication hole32 a is in fluid communication with coolant outlet communication hole 32b.

A second seal member 56 extends circumferentially along the peripheraledge of second metal separator 20 and is integrally joined to thesurfaces 20 a and 20 b of second metal separator 20. On the surface 20 aside of second metal separator 20, the second seal member 56 is providedwith an outside seal 58 a which is disposed on surface 20 a in proximityto the peripheral edge of second metal separator 20, and an inside seal58 b which is inwardly spaced apart from the outside seal 58 a. Theoutside and inside seals 58 a and 58 b are provided on one side ofsecond seal member 56 facing the anode 38.

The outside and inside seals 58 a and 58 b may have any desired shapeselected from a tapered (or lip), trapezoid and semicylindrical shape.The outside seal 58 a is in contact with first planar portion 52 offirst metal separator 18, and the inside seal 58 b is in direct contactwith solid polymer electrolyte membrane 36 of MEA 16.

As shown in FIG. 6, the outside seal 58 a circumscribes oxidant gasinlet communication hole 30 a, coolant outlet communication hole 32 b,fuel gas outlet communication hole 34 b, fuel gas inlet communicationhole 34 a, coolant inlet communication hole 32 a and oxidant gas outletcommunication hole 30 b. The inside seal 58 b circumscribes fuel gasflow channels 44. The peripheral edge of MEA 16 is disposed betweenoutside and inside seals 58 a and 58 b.

On the surface 20 b side of second metal separator 20, the second sealmember 56 is provided with an outside seal (coolant seal) 58 c whichcorresponds to outside seal 58 a, and an inside seal 58 d whichcorresponds to inside seal 58 b (see FIG. 7). The outside and insideseals 58 c and 58 d have the same shape as outside and inside seals 58 aand 58 b.

As shown in FIG. 6, the outside seal 58 a is provided with an inletmanifold 60 which establishes fluid communication between oxidant gasinlet communication hole 30 a and oxidant gas flow channels 42, and anoutlet manifold 62 which establishes fluid communication between oxidantgas outlet communication hole 30 b and oxidant gas flow channels 42.

The inlet manifold 60 is constructed by a plurality of supports 64 whichare formed by cutting off outside seal 58 a at positions spaced apart inthe direction of arrow C and extend in the direction of arrow B.Communication paths for oxidant gas are defined between supports 64. Theoutlet manifold 62 is similarly constructed by a plurality of supports66 which are formed by partially cutting off outside seal 58 a andextend in the direction of arrow B. The supports 66 are in contact withfirst planar portion 52 to define communication paths for oxidant gastherebetween.

The supports 64 of inlet manifold 60 overlie seal laps 68 of outsideseal 58 c while being on the opposite surfaces 20 a, 20 b of secondmetal separator 20. Notably, the seal laps 68 are portions of outsideseal 58 c that overlie supports 64 of outside seal 58 a, with secondmetal separator 20 interposed therebetween.

The outlet manifold 62 is constructed as is the inlet manifold 60. Thesupports 64 and seal laps 70 of outside seal 58 c that overlie eachother on the opposite surfaces 20 a, 20 b of second metal separator 20are set such that the deformation of seals in the stacking directionunder the load applied in the stacking direction is substantiallyequalized (see FIG. 6).

As shown in FIG. 7, the surface 20 b of second metal separator 20 isprovided with an inlet manifold 72 which establishes fluid communicationbetween coolant inlet communication hole 32 a and coolant flow channels46, and an outlet manifold 74 which establishes fluid communicationbetween coolant outlet communication hole 32 b and coolant flow channels46. The inlet manifold 72 is constructed by a plurality of supports 76which are spaced apart in the direction of arrow C, extend in thedirection of arrow B, and constitute outside and inside seals 58 c and58 d. The outlet manifold 74 is similarly constructed by a plurality ofsupports 78 which are spaced apart in the direction of arrow C, extendin the direction of arrow B, and constitute outside and inside seals 58c and 58 d.

The inlet manifold 72 overlies seal laps 80 a and 80 b constitutingoutside and inside seals 58 a and 58 b on surface 20 a, with secondmetal separator 20 interposed therebetween.

Similarly, supports 78 constituting outlet manifold 74 overlie seal laps82 a and 82 b of outside and inside seals 58 a and 58 b, while being onopposite surfaces 20 a and 20 b of second metal separator 20, as shownin FIG. 7.

As shown in FIG. 7, on the surface 20 b of second metal separator 20, aninlet manifold 84 and an outlet manifold 86 are provided in proximity tofuel gas inlet communication hole 34 a and fuel gas outlet communicationhole 34 b, respectively. The inlet manifold 84 is provided with aplurality of supports 88 arranged in the direction of arrow C, and theoutlet manifold 86 is similarly provided with a plurality of supports 90arranged in the direction of arrow C.

The supports 88 of inlet manifold 84 overlie seal laps 92 a and 92 b ofoutside and inside seals 58 a and 58 b, with second metal separator 20interposed therebetween. Similarly, the supports 90 of outlet manifold86 overlie seal laps 94 a and 94 b of outside and inside seals 58 a and58 b, with second metal separator 20 interposed therebetween.

The inlet manifold 84 and seal laps 92 a, 92 b, and the outlet manifold86 and seal laps 94 a, 94 b are set such that the deformation of sealsin the stacking direction under the load applied in the stackingdirection is substantially equalized. Specifically, the inlet manifold84 is constructed as is the inlet manifold 72. A plurality of feed holes96 and discharge holes 98 are formed in proximity to inlet and outletmanifolds 84 and 86 and disposed outward of inside seal 58 d. The feedholes 96 and discharge holes 98 are formed throughout the separatorinward of inside seal 58 b on the surface 20 a side of second metalseparator 20 and at the inlet and outlet sides of fuel gas flow channels44 (see FIG. 6).

Although outside seal 58 c is formed as a coolant seal on the surface 20b of second metal separator 20 in the illustrated embodiment, theinvention is not limited thereto. Such a coolant seal may be formed onthe surface 18 b of first metal separator 18.

EXAMPLE

Examples and Comparative Examples are given below for furtherillustrating the invention, but the invention is not limited thereto.All parts are by weight. Mw is weight average molecular weight.

Example 1

70 parts of dimethylpolysiloxane #1 capped with dimethylvinylsiloxy atboth ends and having a Mw of 26,000 and a vinyl content of 0.000088mol/g and 10 parts of gum-like dimethylpolysiloxane #2 capped withdimethylvinylsiloxy at both ends, containing vinyl on side chains, andhaving a Mw of 370,000 and a vinyl content of 0.00052 mol/g were mixedwith 30 parts of fumed silica having a BET specific surface area of 200m²/g (Aerosil 300, Nippon Aerosil Co., Ltd.), 6 parts ofhexamethyldisilazane, and 2.0 parts of water at room temperature for 30minutes. The mixture was heated at 150° C., agitated at the temperaturefor 3 hours, and cooled. The mixture was further combined with 50 partsof dimethylpolysiloxane #1 and milled for 30 minutes, yielding asilicone rubber base. To 160 parts of the silicone rubber base wereadded 3.3 parts (giving [Si—H/vinyl]=1.5 in molar ratio) ofmethylhydrogenpolysiloxane #3 capped with dimethylsiloxy at both ends,containing Si—H groups at ends and on side chains, and having a degreeof polymerization of 20 and a Si—H content of 0.0072 mol/g as acrosslinking agent and 0.05 part of ethynyl cyclohexanol as a reactionregulator. Continued milling for 15 minutes gave a mixture. Onmeasurement at 25° C., the mixture had a viscosity of 46 Pa-s at 10 s⁻¹and 59 Pa-s at 0.9 s⁻¹.

A silicone rubber composition was prepared by combining 100 parts of themixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %),press cured at 120° C. for 10 minutes, and post cured in an oven at 200°C. for 4 hours. The cured sample was measured for hardness according toJIS K6249 and for compression set after heating at 150° C. for 70 hours,with the results shown in Table 1. A molding test was performed byinsert molding the silicone rubber composition on a stainless steelplate to form a seal having the structure shown in FIG. 5.

Example 2

43 parts of dimethylpolysiloxane #4 capped with dimethylvinylsiloxy atboth ends and having a Mw of 18,000 and a vinyl content of 0.00011mol/g, 20 parts of dimethylpolysiloxane #5 capped with trimethylsiloxyat both ends, containing vinyl on side chains and having a Mw of 42,000and a vinyl content of 0.00007 mol/g and 12 parts of gum-likedimethylpolysiloxane #6 capped with dimethylvinylsiloxy at both ends,containing vinyl on side chains and having a Mw of 450,000 and a vinylcontent of 0.000022 mol/g were mixed with 35 parts of fumed silicasurface treated to be hydrophobic and having a BET specific surface areaof 260 m²/g (Rheorosil DM30S, Tokuyama Co., Ltd.), 5 parts ofhexamethyldisilazane, and 1.0 part of water at room temperature for 30minutes. The mixture was heated at 150° C., agitated at the temperaturefor 3 hours, and cooled. The mixture was further combined with 50 partsof dimethylvinylsiloxy-endcapped dimethylpolysiloxane #4 and milled for30 minutes, yielding a silicone rubber base. To 160 parts of thesilicone rubber base were added 3.3 parts (giving [Si—H/vinyl]=2.0 inmolar ratio) of methylhydrogenpolysiloxane #3 (in Example 1, degree ofpolymerization 20 and Si—H content 0.0072 mol/g) as a crosslinking agentand 0.05 part of ethynyl cyclohexanol as a reaction regulator. Continuedmilling for 15 minutes gave a mixture. On measurement at 25° C., themixture had a viscosity of 81 Pa-s at 10 s⁻¹ and 109 Pa-s at 0.9 s⁻¹.

A silicone rubber composition was prepared by combining 100 parts of themixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %)before it was measured for hardness and compression as in Example 1,with the results shown in Table 1. A molding test was performed byinsert molding the silicone rubber composition on a stainless steelplate to form a seal having the structure shown in FIG. 5.

Comparative Example 1

80 parts of dimethylpolysiloxane #7 capped with dimethylvinylsiloxy atboth ends, containing vinyl on side chains, and having a Mw of 49,000and a vinyl content of 0.000081 mol/g was mixed with 30 parts of fumedsilica having a BET specific surface area of 200 m²/g (Aerosil 300,Nippon Aerosil Co., Ltd.), 6 parts of hexamethyldisilazane, and 2.0parts of water at room temperature for 30 minutes. The mixture washeated at 150° C., agitated at the temperature for 3 hours, and cooled.The mixture was further combined with 50 parts of dimethylpolysiloxane#7 and milled for 30 minutes, yielding a silicone rubber base. To 160parts of the silicone rubber base were added 2.2 parts (giving[Si—H/vinyl]=1.5 in molar ratio) of methylhydrogenpolysiloxane #3 (inExample 1, degree of polymerization 20 and Si—H content 0.0072 mol/g) asa crosslinking agent and 0.05 part of ethynyl cyclohexanol as a reactionregulator. Continued milling for 15 minutes gave a mixture. Onmeasurement at 25° C., the mixture had a viscosity of 52 Pa-s at 10 s⁻¹and 65 Pa-s at 0.9 s⁻¹.

A silicone rubber composition was prepared by combining 100 parts of themixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %),press cured at 120° C. for 10 minutes, and post cured in an oven at 200°C. for 4 hours. The cured sample was measured for hardness according toJIS K6249 and for compression set after heating at 150° C. for 70 hours,with the results shown in Table 1. A molding test was performed byinsert molding the silicone rubber composition on a stainless steelplate to form a seal having the structure shown in FIG. 5.

Comparative Example 2

60 parts of dimethylpolysiloxane #8 capped with dimethylvinylsiloxy atboth ends, containing vinyl on side chains, and having a Mw of 58,000and a vinyl content of 0.000092 mol/g and 20 parts of gum-likedimethylpolysiloxane #6 capped with dimethylvinylsiloxy at both ends,containing vinyl on side chains and having a Mw of 450,000 and a vinylcontent of 0.000022 mol/g were mixed with 40 parts of fumed silicasurface treated to be hydrophobic and having a BET specific surface areaof 260 m²/g (Rheorosil DM30S, Tokuyama Co., Ltd.), 5 parts ofhexamethyldisilazane, and 1.0 part of water at room temperature for 30minutes. The mixture was heated at 150° C., agitated at the temperaturefor 3 hours, and cooled. The mixture was further combined with 50 partsof dimethylpolysiloxane #4 and milled for 30 minutes, yielding asilicone rubber base. To 160 parts of the silicone rubber base wereadded 3.3 parts (giving [Si—H/vinyl]=2.0 in molar ratio) ofmethylhydrogenpolysiloxane #3 (in Example 1, degree of polymerization 20and Si—H content 0.0072 mol/g) as a crosslinking agent and 0.05 part ofethynyl cyclohexanol as a reaction regulator. Continued milling for 15minutes gave a mixture. On measurement at 25° C., the mixture had aviscosity of 215 Pa-s at 10 s⁻¹ and 335 Pa-s at 0.9 s⁻¹.

A silicone rubber composition was prepared by combining 100 parts of themixture with 0.1 part of a platinum catalyst (Pt concentration 1 wt %)before it was measured for hardness and compression as in Example 1,with the results shown in Table 1. A molding test was performed byinsert molding the silicone rubber composition on a stainless steelplate to form a seal having the structure shown in FIG. 5.

TABLE 1 Example Comparative Example 1 2 1 2 Hardness, Durometer A 42 4843 49 Compression set in air, % 5.5 6.2 9.1 8.8 Molding test, voidedsamples 0% 0% 0% 80%

Below described is the molding test for inspecting whether an integralseal-separator having the structure shown in FIG. 5 is moldable from thesilicone rubber composition. A stainless steel substrate having a primercoated thereon (Primer No. 101A/B, Shin-Etsu Chemical Co., Ltd., airdrying+150° C.×30 min baking) was furnished as a separator. By insertmolding, the silicone rubber composition was molded and cured around thesubstrate in a mold at a temperature of 150° C. for 5 minutes. Afterremoval from the mold, the sample was post cured at 200° C. for 4 hours.The molded seal was inspected for the presence of voids and otherunfilled defects. Provided that samples without such voids pass thetest, a percentage of voided samples is shown in Table 1.

Japanese Patent Application No. 2007-317475 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A separator seal for use in polymer electrolyte fuel cells which isformed of a sealing composition in the cured state, said sealingcomposition comprising (A) 100 parts by weight of a liquidorganopolysiloxane containing at least two alkenyl groups each attachedto a silicon atom in a molecule and having a weight average molecularweight of less than 100,000, (B) 3 to 30 parts by weight of a gum-likeorganopolysiloxane having a weight average molecular weight of at least150,000, (C) 0.5 to 20 parts by weight of an organohydrogenpolysiloxanecontaining at least three hydrogen atoms each attached to a silicon atomin a molecule, (D) 5 to 30 parts by weight of fumed silica having a BETspecific surface area of 50 to 400 m²/g, and (E) a catalytic amount ofan addition reaction catalyst, said composition having a viscosity of 20to 200 Pa-s at 25° C. and a shear rate of 10 s⁻¹.
 2. The separator sealof claim 1 wherein the organopolysiloxane (A) has a weight averagemolecular weight of 10,000 to 80,000, at least 90 mol % of the entireorganic groups attached to silicon atoms being methyl.
 3. The separatorseal of claim 1 wherein a molar ratio of Si—H groups in component (C) tototal alkenyl groups in components (A) and (B) is between 0.8:1 and5.0:1.
 4. The separator seal of claim 1 wherein said composition at 25°C. has a viscosity V(10) at a shear rate of 10 s⁻¹ and a viscosityV(0.9) at a shear rate of 0.9 s⁻¹, which fall in the range:1.0<V(0.9)/V(10)<2.5.
 5. The separator seal of claim 1 wherein thegum-like organopolysiloxane (B) contains 1.0×10⁻⁶ to 1.0×10⁻³ mol/g ofalkenyl groups.
 6. A separator for use in polymer electrolyte fuel cellscomprising a substrate comprising a metal thin plate or a conductivepowder and a binder and a seal formed at a periphery on at least onesurface of the substrate, said seal comprising the separator seal of anyone of claims 1 to 5.